Sandwich panels are used in many aircraft interior applications, such as floors, sidewalls, ceilings and stowage compartments. These types of sandwich panels may be used in similar applications in other types of transport vehicles. In addition to providing a finishing function, the sandwich panels need to have adequate weight and thickness and possess certain mechanical properties and have sufficient fire resistance.
Of particular interest to the applicant is the use of sandwich panels in aircraft interiors. Consequently, the following description focuses on the manufacture of novel sustainable, or environmentally friendly, sandwich panels in aircraft interiors. It will be abundantly clear that the present invention may extend to the manufacture of sandwich panels in general. Such general sustainable sandwich panels enjoy far greater applicability than just aircraft interiors and would not require any modification.
Current aircraft interior panels are sandwich structures comprising a core sandwiched between outer skins. The materials used in these panels are chosen primarily for their fire resistant properties. For commercial airliners, there are strict regulations governing the fire resistant properties of the materials used in the cabin, along with limits as to the heat and smoke released during combustion of such materials. This has led to the widespread use of glass fibre-reinforced composites based on phenolic resins in conventional aircraft interior parts. In addition to their appropriate fire resistance, the panels based on these composite materials may be moulded into complex shapes, they have a high strength-to-weight ratio, appropriate flexural strength and impact resistance, have low maintenance costs and are generally easily installed.
In general, phenolic resins and glass fibre pre-pregs comprise the outer skins of such panels. Alternatively, skins may be made from a composite of glass fibre with epoxy or carbon fibre with epoxy. All these skin materials have known environmental limitations. Phenolic resins are regarded as highly noxious and can cause skin problems, such as dermatitis. Glass fibres cause irritation of the skin, eyes and upper respiratory system producing skin eruption similar in appearance to poison ivy, pneumoconiosis and silicosis. If ingested, glass fibres can also cause gastrointestinal conditions.
The core of a conventional panel is usually formed from a Nomex® honeycomb that contains aramide fibres. These fibres are a heat-resistant synthetic fibre, but have a known disadvantage in that upon fracturing, they produce small fibrils that are harmful to the lungs and cause skin irritation.
The use of such noxious skin and core materials presents difficulties during manufacturing, while heating the resins and where fibres may be exposed after curing, such that careful handling is required. Personal protective equipment is therefore required during manufacturing such panels. This does not apply once the part is made and installed on the aircraft. However, more significant issues arise at the end of the service life of the aircraft where it is scrapped and parts are disposed of. This is of course true for removal and disposal of interior panels at any stage of the aircraft's life, for example during a refit or conversion process. Moreover, the noxious nature of the materials makes the panels poor candidates for recycling and so often end up being sent for burial at landfill. They do not leach but still constitute harmful residues. This is contrary to the aerospace industries current drive for products that achieve a better environmental performance.
The ideal situation would therefore be that in which the sandwich panels are more environmentally friendly while maintaining an excellent technical performance. For example, sandwich panels that are easier to recycle or dispose would be extremely advantageous. An improvement on conventional sandwich panels has been described in EP-A-2,463,083. This document discloses the use of sustainable materials in sandwich panels, namely a sustainable sandwich panel comprising skins formed from natural fibres set within inorganic thermoset or thermoplastic resins and a core formed from fire resistant balsa wood, a fire resistant paper honeycomb or a fire resistant thermoplastic foam.
The present invention provides an improved method of manufacturing such panels comprising skins formed of natural fibres set within inorganic thermoset resin that sandwich a foam core. The improved method of manufacture leads to very much reduced production times for the panels.